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We investigated the temperature-dependent magnetoresistance of granular (Ga,Mn)As/MnAs hybrids grown on (100) GaAs in different transport geometries. The observed magnetoresistance effects are much bigger than for a corresponding (Ga,Mn)As reference sample without MnAs nanoclusters. We find that the magnetoresistance effects depend strongly on the chosen transport geometry. When the external field is perpendicular to the sample plane the effects are largest. The smallest effects occur when the external field is in the sample plane and parallel to the current. Furthermore, we have established by ferromagnetic resonance studies that the magnetic properties of the ensemble of ferromagnetic MnAs nanoclusters is similar for the magnetic field orientations studied. Therefore, the observed anisotropy of the magnetoresistance mainly reflects the difference in current path through the sample which leads to a variation of the degree of interaction between the free carriers in the matrix and nanoclusters.

An important parameter of the ferromagnetism of (Ga,Mn)As alloys is the average p-d exchange integral N 0 ␤ since the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction parameter J RKKY ϰ ͑N 0 ␤͒ 2 . We prove, by a detailed study of p-type Ga 1−x Mn x As and n-type Ga 1−x Mn x As: Te ͑x Ͻ 0.005͒ in the paramagnetic phase, that the magnitude and sign of N 0 ␤ are strongly dependent on the local electronic configuration of the Mn ions. We find N 0 ␤ Ͼ 0 (ferromagnetic coupling) in p-type samples where the local Mn configuration is Mn Ga 0 ͑3d 5 + hole͒. In n-type samples, N 0 ␤ Ͻ 0 (antiferromagnetic coupling) with the local Mn configuration being Mn Ga − ͑3d 5 ͒. The average N 0 ␤ in (Ga,Mn)As is not a material constant as widely believed. It can be tuned and can even change its sign, depending on the doping conditions of the Mn-containing layer.

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